I. Field of the Invention
The present invention relates to a control method for antilock braking systems capable of precluding the locking of the wheels of a motor vehicle on sudden application of the brake and to a measurement device of the vehicle wheel operating force capable of using antilock braking systems.
II. Prior Art
If the wheels of a motor vehicle are locked upon application of the brake of the motor vehicle, rotation of the tire of vehicle is braked and it is followed by tire slipping on the road surface.
Such a condition means the less of vehicle controllability. Antilock braking systems (ABS) are control systems capable of preventing the tire from locking during braking process.
Generally the conventional antilock braking system for motor vehicles employs a system such that the braking action is automatically controlled in accordance with speed of the vehicle wheel. The vehicle wheel speed is measured with use of a wheel speed sensor. This wheel speed sensor has a structure measuring rotational velocity of the vehicle wheel by detecting the rotation speed of a geared hub attached on the axle of wheel with the use of a magnetic pick-up.
Control procedure of the conventional antilock braking system is as follows: when the braking is suddenly applied, the locking of the wheel is detected by rapidly decreasing of the wheel speed. Then the control system reduces braking pressure. Thereafter, when the wheel speed recovers by the braking pressure reduction, the braking is again applied. In such a control method, however, the effect of brake control is lost in fact during a period from the occurrence of the wheel locking to the resumption of the braking process. Consequently the controllability becomes unstable and it involves the longer stopping distance for the vehicle.
In another conventional antilock braking systems, the control of the system is commenced by using an arbitrarily chosen control method and parameter settings. Thereafter the frequency of the tire slipping is counted during controlling and it estimates the friction coefficient of the road surface on the basis of the counted slip frequency. When the slip frequently occurs, the control method is changed into the method suitable for the road surface having lower friction coefficient, and when there is no slip, it is switched into the method suitable for the road surface having higher friction coefficient. However, such a control method causes to increase in the loss of the breaking force during the tire slipping and results in the longer stopping distance for the vehicle.
It is possible to control operations of antilock braking systems by using a measured value of a stress applied on the vehicle wheel. There are a photoelasticity method, a brittle coat method, a holography method, a strain gauge method, etc. as methods measuring stress about a structure. Generally, the strain gauge method is widely used, but it is difficult to detect an intended stress value because it receives other several forces than the stress to be measured. In particular, there is no method detecting side forces.